Positron emission tomography (PET) has been widely used in medicine for diagnosis and other purposes. An object, such as a patient, may be scanned with a PET system to obtain PET datasets. For a ring tomography, the raw line-of-response (LOR) may be unevenly spaced. The LORs near the center of the PET ring may be wider than those far away from the center of the PET ring. Conventional reconstruction algorithms may act on data which be pre-processed into corrected, evenly spaced histograms; however, such pre-processing may corrupt the characteristics, e.g., Poisson statistics. The axial length of the detector may be smaller than the length of the patient under examination, besides, at one bed position, an image obtained may be with a non-uniform SNR in different region.